The reason why some sessions aren't showing up in the Add Insurance Payment page is because the sessions you're looking for are set with the wrong Billing Type. You need these sessions to be set with the billing type Insurance but they're set up as Self-Pay.
To resolve this, you will need to go to the client's Billing Page. Once there, place your mouse cursor on the session fee for the sessions in question > click on the Edit Button > change billing type to Insurance.
Once you've done this, you will see that session(s) appear in the Add Insurance Payment page. If you need help adding an insurance payment, click HERE.